A Low RF-Band Impedance Spectroscopy Based Sensor for In Situ, Wireless Soil Sensing

A novel on-board solution for a robust, accurate and self-calibrating soil moisture and nutrient sensor with inbuilt wireless transmission and reception capability is presented. The sensor can make real time and accurate measurement of subsurface soil moisture and nutrients and is ideally suited to act as a node in a network spread over a large area. It works on the principle of impedance spectroscopy. Impedance measurement at multiple frequencies is done by comparing the amplitude and phase of signals incident on and reflected from the soil in proximity of the sensor. A distributed transmission line model for the on-board connections is employed to improve the accuracy of measurements. Variations in temperature and surroundings are accounted by realizing a built-in self-calibrating mechanism which operates on the standard short-open-load technique. This also accounts for the parasitic impedance of the board in the measurements to minimize the errors. Measurements of both real and imaginary parts of soil impedance at multiple frequencies give the sensor an ability to detect variations in ionic concentrations other than soil moisture content. A switch-controlled multiple power mode transmission and reception is provided to support highly energy efficient medium access control.